As well known by people involved in the art, chromatographic systems rely on the use of valves to allow reproducible sample introduction and various column switching schemes.
For the last forty years, many people have designed diaphragm valves for chromatography. Such diaphragm valves have been used in many commercially available gas chromatographs. They are apt to be integrated more easily in a gas chromatograph due to their physical size and since the actuator is embedded in the valve itself. These characteristics make them attractive for gas chromatograph manufacturers.
For example, international application no. PCT/CA2008/002138, filed Dec. 5, 2008 by the present applicants and published as WO2009/073966, discloses such a diaphragm-sealed valve. In addition, U.S. Pat. Nos. 7,216,528 and 7,503,203 issued to the present applicants May 15, 2007 and Mar. 17, 2009, respectively, discloses other types of diaphragm-sealed valves.
Referring now to FIG. 1 (PRIOR ART), there is shown an example of a typical diaphragm-sealed valve as known in the art. The valve 1 is provided with a top block 2 having an interface 4 and a plurality of ports 6. Each of the ports 6 opens at the interface 4 and has an inclined thread passage 8 to connect various analytical fitting and tubing (not shown). At the bottom of the inclined thread passage 8, there is a conduit 10 extending in the top block 2 and opening at the interface 4. The ports 6 are arranged on a circular line on the interface 4 of the top block 2. The interface 4 is advantageously flat and polished to minimize leaks between ports and from the ambient atmosphere. The valve 1 is also provided with a bottom block 12 and a diaphragm 14, which is generally made of polyimide, Teflon or other polymer material. The diaphragm 14 is positioned between the top block interface 4 and the bottom block 12, and has a recess 18 therein extending along the circular line formed by the ports 6 and biased away from the interface 4 of the top block 2. The recess 18 in the diaphragm 14 sits in a matching recess 20 made in the bottom block 12, thereby allowing some clearance for fluid circulation between adjacent ports 6.
The valve 1 is also provided with a plurality of plungers 16 mounted in the bottom block 12, each being respectively arranged to be able to compress the diaphragm 14 against the top block 2 at a position located between two of the ports 6. Preferably, as illustrated, when the valve is at rest, three plungers 16 are up while the other three are down. When the plungers are up, they compress the diaphragm 14 against the top block 2 and close the conduits made by the diaphragm recess 18, so that fluid circulation is blocked. The bottom block 12 keeps the plungers 16 and the actuating mechanism in position.
It is common to designate a portion of the plungers 16 as “normally open” and another portion as “normally closed”. A normally open plunger 16 is biased downwards, i.e. away from the diaphragm 14, and therefore normally allows fluid circulation between the two adjacent ports 6. A normally closed plunger 16 is biased upwards, i.e. towards the diaphragm 14, and therefore blocks fluid circulation between the two adjacent ports 6. A user may actuate the valve 1 in order to alter the positions of the plungers 16, for example by sliding upwards and downwards the normally open and closed plungers 16, respectively.
However, it has been found that prolonged deformation of a diaphragm by a normally closed plunger, for example during storage or shipping, can damage the diaphragm. This damage can both shortens the diaphragm's life and compromise the system's performance. A damaged diaphragm can also increase the leak rate from port to port. When the pressure drop on the valve's ports differs from port to port, the pressure and flow may vary in the system. This causes detrimental effects on column performance and detector baseline.
There is therefore a need for an improved diaphragm-sealed valve.